X-ray imaging apparatus and measurement method

ABSTRACT

An X-ray imaging apparatus which obtains an X-ray image, the apparatus includes: an imaging unit including a plurality of detecting elements adapted to convert X-rays generated by an X-ray generating apparatus which outputs or stops X-rays in accordance with an operation instruction into an image signal; and an obtaining unit adapted to obtain an operation start timing of the X-ray generating apparatus based on an image signal output from the imaging unit and obtain a difference between a timing of an operation instruction to the X-ray generating apparatus and the operation start timing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an X-ray imaging apparatus and ameasurement method and, more particularly, to a technique for measuringdelays associated with X-ray output operation.

2. Description of the Related Art

An X-ray imaging apparatus is known, which uses a flat panel detectorincluding a two-dimensional array of detecting elements and TFTs formedfrom an amorphous silicon and polysilicon, as materials that aredeposited and formed on a glass substrate. Such apparatuses vary intype, but are generally configured such that when X-rays strike the flatpanel detector, the phosphor wavelength-converts X-rays into visiblelight. The detecting elements convert the converted light into chargesand store the charges. When the TFTs are turned on for each row, thecharges stored in the flat panel detector are sequentially read andconverted into pixel values. Using such a flat panel detector, an X-rayimaging apparatus generates an image according to the intensitydistribution of X-rays, on the flat panel detector, which aretransmitted through an object placed between an X-ray generatingapparatus (X-ray source) which emits X-rays and the flag panel detector.

Recently, flat panel detectors capable of capturing moving images aswell as still images have been developed.

The following can influence the quality of moving images when capturinga moving image performed by repeatedly capturing still images at a highspeed:

-   -   the difference between the timing at which the X-ray generating        apparatus receives a signal indicating X-ray exposure and the        timing at which the X-ray generating apparatus starts X-ray        exposure in accordance with the signal (X-ray exposure start        delay time), and    -   the difference between the timing at which the X-ray generating        apparatus receives a signal indicating when X-ray exposure has        stopped and the timing at which the X-ray generating apparatus        stops X-ray exposure in accordance with the signal (X-ray        exposure stop delay time).        This point will be described with reference to FIGS. 7 to 9.        FIG. 7 is a timing chart showing the relationship between an        X-ray exposure signal, X-ray intensity, and read in the moving        image capturing mode.

The flat panel detector alternately repeats storing and reading chargesoriginating from X-ray exposure. Referring to FIG. 7, “read” indicatesperiods during which charges are read from the flat panel detector, “Hi”indicates a period during which charges are read, and “Lo” indicates aperiod during which no charge is read (charges are stored in the caseshown in FIG. 7).

When capturing an image by reading charges, offset correction isgenerally performed. A technique is known where offset correction isperformed for a moving image capturing apparatus including a flat paneldetector and driving the apparatus (Japanese Patent Laid-Open No.2002-301053). FIG. 7 shows a timing chart when an X-ray image isgenerated by reading charges twice per X-ray exposure, and subtractingan image read without X-ray exposure from an image read after X-rayexposure.

Referring to FIG. 7, “X-ray exposure signal” indicates the transition ofan X-ray exposure signal supplied to the X-ray generating apparatus,with a Hi period of the X-ray exposure signal indicating X-ray exposure,and a Lo period of the signal indicating the stop of X-ray exposure. TheX-ray generating apparatus (not shown) starts X-ray exposure when theX-ray exposure signal goes Hi, and stops X-ray exposure when the X-rayexposure signal goes Lo. It takes a certain time from the instant theX-ray exposure signal goes Hi to the instant X-ray exposure actuallystarts (X-ray exposure start delay time). It also takes a certain timefrom the instant the X-ray exposure signal goes Lo to the instant X-rayexposure actually stops (X-ray exposure stop delay time). Referring toFIG. 7, “X-ray intensity” indicates the transition of the intensity ofX-rays actually output from the X-ray generating apparatus. Referring toFIG. 7, reference symbol Ta denotes an X-ray exposure start delay time;and Tb, an X-ray exposure stop delay time. In addition, X-ray exposureis performed during storage of charges.

If the frame rate in moving image capturing is high, the magnitudes ofthe X-ray exposure start delay time and X-ray exposure stop delay timebecome large relative to the frame interval. FIG. 8 is a timing chartshowing an X-ray exposure signal, X-ray intensity, and read when theframe rate in the moving image capturing mode is high.

As the frame rate in moving image capturing increases, since the chargeread time is constant, the charge storage period decreases. Actual X-rayexposure may overlap reading of charges because the X-ray exposure isaccompanied by an X-ray exposure start delay time and an X-ray exposurestop delay time. Referring to FIG. 8, the hatched portion indicates aportion where actual X-ray exposure overlaps a charge read. Referencesymbol Tc denotes an overlap time.

It is generally necessary to emit a predetermined dose of X-rays tocapture an X-ray image. If, therefore, X-ray exposure overlaps a chargeread, since the intensity of emitted X-rays is not reflected in thestored charges, the quality of an X-ray image deteriorates.

To emit the predetermined dose of X-rays, it is conceivable to secure apredetermined period of time during which an X-ray exposure signal isset Hi or shorten a “Hi” period of the X-ray exposure signal andincrease an X-ray tube current or X-ray tube voltage for the X-raygenerating apparatus. As in the latter case, an increase in X-ray tubecurrent in the X-ray generating apparatus will lead to an increase incost, whereas an excessive increase in X-ray tube voltage will lead to adecrease in the contrast of a captured X-ray image. For this reason, toemit the predetermined dose of X-rays, the technique of securing apredetermined period of time during which the X-ray exposure signal isset Hi is generally used. In this case, in order to increase the framerate in moving image capturing, it is necessary to minimize the timeinterval from the end of charge reading to the start of X-ray exposureand the time interval from the end of X-ray exposure and the start ofcharge reading.

FIG. 9 is a timing chart showing the relationship between an X-rayexposure signal, X-ray intensity, and read when X-ray exposure startsconcurrently with the end of charge reading, and charge reading startsconcurrently with the end of X-ray exposure.

Referring to FIG. 9, the X-ray exposure start delay time Ta and an X-rayexposure stop delay time Tb are measured in advance. The X-ray exposuresignal is set Hi the measured time Ta earlier than the timing of the endof charge reading. In addition, the X-ray exposure signal is set Lo themeasured time Tb earlier than the timing of the start of charge reading.Performing this control can maximize the frame rate without making anactual X-ray exposure period overlap a charge read period.

In general, a flat panel detector can be connected to various types ofX-ray generators depending on the region to be imaged or the imagingpurpose. However, different X-ray generators differ in X-ray exposurestart delay time and X-ray exposure stop delay time. For this reason, itis necessary to measure an X-ray exposure start delay time and an X-rayexposure stop delay time in advance for each apparatus to be used.

Japanese Patent Laid-Open No. 62-276798 discloses an arrangement formeasuring an X-ray exposure start delay time and an X-ray exposure stopdelay time, and correcting the X-ray exposure start timing and the X-rayexposure stop timing by correcting the measured delay times. Inaddition, Japanese Patent Laid-Open No. 2004-166728 discloses anarrangement in which an X-ray detector different from a flat paneldetector is provided outside the flat panel detector to measure a readtiming.

With regard to the arrangement disclosed in Japanese Patent Laid-OpenNo. 62-276798, there is no description about a specific technique ofmeasuring the X-ray exposure start delay time Ta and the X-ray exposurestop delay time Tb. It is generally necessary to additionally prepare anX-ray detector for measuring an actual X-ray intensity and to measure anoutput from the X-ray detector and an X-ray exposure signal and a readsignal with an oscilloscope or the like. This measurement requires muchtime and labor.

In addition, the arrangement disclosed in Japanese Patent Laid-Open No.2004-166728 requires an additional X-ray detector outside the flat paneldetector to measure the X-ray exposure start delay time Ta and the X-rayexposure stop delay time Tb, and hence has the problem of high cost.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the aboveproblems, and aims to provide a technique of easily measuring delaytimes occurring at the start and stop of X-ray exposure at a low costwithout providing any special arrangement.

According to one aspect of the present invention, an X-ray imagingapparatus which obtains an X-ray image, the apparatus includes: animaging unit including a plurality of detecting elements adapted toconvert X-rays generated by an X-ray generating apparatus which outputsor stops X-rays in accordance with an operation instruction into animage signal; and an obtaining unit adapted to obtain an operation starttiming of the X-ray generating apparatus based on an image signal outputfrom the imaging unit and obtain a difference between a timing of anoperation instruction to the X-ray generating apparatus and theoperation start timing.

According to another aspect of the present invention, a method ofmeasuring a delay time of X-ray exposure by an X-ray imaging apparatuswhich obtains an X-ray image, the method includes the steps of: causingan X-ray generating apparatus to change an X-ray output state inaccordance with an operation signal; sequentially reading image signalsfrom an imaging unit adapted to convert the X-rays into image signals;and measuring a delay of operation of the generating apparatus relativeto the operation signal by analyzing the image signal.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the arrangement of an X-rayimaging apparatus according to an embodiment of the present invention;

FIG. 2 is a schematic plan view showing the arrangement of a flat paneldetector according to the embodiment of the present invention;

FIGS. 3, 4, 5, and 6 are timing charts for measurement of X-ray exposuredelay times according to the embodiment of the present invention; and

FIGS. 7, 8, and 9 are timing charts associated with X-ray exposure andcharge reading in moving image capturing.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention will be described in detailbelow with reference to the accompanying drawings.

(Arrangement of X-Ray Imaging Apparatus)

FIG. 1 is a schematic block diagram of an X-ray imaging apparatusaccording to this embodiment. Reference numeral 101 denotes an X-raygenerator which controls X-ray exposure; 102, an X-ray tube 102 whichemits X-rays; 100, an object to be examined (object to be imaged); 103,a flat panel detector for detecting X-rays transmitted through theobject 100; 104, a display unit for displaying an X-ray image read fromthe flat panel detector 103; and 105, a control unit which controls theoverall X-ray imaging apparatus.

The control unit 105 is connected to the X-ray generator 101 via anX-ray exposure signal. When the control unit 105 controls an X-rayexposure signal (to Hi or Lo), the X-ray generator 101 starts or stopsX-ray exposure. The control unit 105 performs control to execute X-rayexposure once in the still image capturing mode and continuously emitX-rays in a pulse manner in the moving image capturing mode. Note thatwhen measuring delay times in X-ray exposure, imaging is performed inthe absence of an object to be examined as an object to be imaged.

The control unit 105 is connected to the flat panel detector 103 via aread start signal, and controls the read start signal (to Hi or Lo) tomake the flat panel detector 103 start reading. The control unit 105 isalso connected to the flat panel detector 103 via a MODE signal line,and controls the MODE signal line (to Hi or Lo) to switch between thestill image capturing mode and the moving image capturing mode. In thisembodiment, turning on the gate lines of the flat panel detector 103 foreach row will sequentially read charges from the flat panel detector103. In this manner, the embodiment obtains an image by reading chargesfrom detecting element constituting the flat panel detector for eachrow. The scanning speed associated with charge reading is constant, andis known in advance.

In addition, the control unit 105 includes a timing control unit 106which controls the X-ray exposure timing and the read timing of the flatpanel detector 103. The timing control unit 106 generally performscontrol to perform X-ray exposure during a storage period of the flatpanel detector 103. When measuring an X-ray exposure start delay time Taand an X-ray exposure stop delay time Tb, in particular, the timingcontrol unit 106 controls the X-ray generator 101 to start and stopX-ray exposure during the read operation of the flat panel detector 103.

The control unit 105 also includes a delay time measuring unit (X-rayexposure delay time measuring unit) 107 which measures a delay timeassociated with X-ray exposure by analyzing an image obtained by X-rayexposure during read operation of the flat panel detector 103. The delaytime measuring unit 107 includes a start delay measuring unit (X-rayexposure start delay time measuring unit) 108 and a stop delay measuringunit (X-ray exposure stop delay time measuring time unit) 109. The startdelay measuring unit 108 measures the X-ray exposure start delay timefrom the instant a signal indicating X-ray exposure is input to theinstant X-rays are actually emitted. The stop delay measuring unit 109measures the X-ray exposure stop delay time from the instant a signalindicating the stop of X-ray exposure is input to the instant X-rayexposure actually stops. As will be described later, this embodimentmeasures the delay time of X-ray exposure based on the position of theboundary between a region where there is a change in pixel value in animage obtaining by emitting uniform X-rays and performing scanning at aconstant speed and a region where there is no change in pixel value inthe image, and a scanning speed associated with charge reading.

(Arrangement of Flat Panel Detector)

FIG. 2 is a schematic plan view of the flat panel detector 103. The flatpanel detector 103 in FIG. 2 has an arrangement in which detectingelements for converting received X-ray signals into charges are providedin a matrix form. In practice, the number of detecting elements is about2000×2000. The charges converted from X-ray signals by the detectingelements are respectively stored in the corresponding capacitors.

Referring to FIG. 2, the X-axis is set in the longitudinal direction onthe drawing surface. Assume that in this embodiment, the read sequenceof the detecting elements coincides with the direction in whichX-coordinates increase (in the downward direction in this embodiment).In addition, X1 represents a coordinate of the uppermost row of the flatpanel detector 103, and X6 represents a coordinate of the lowermost row.Coordinates X2 to X5 are coordinates between the coordinates X1 and X6.The embodiment reads charges from the detecting elements constitutingthe flat panel detector row by row at a constant speed. The coordinatesX2 and X4 respectively correspond to the positions of a leading edge andtrailing edge of an X-ray exposure signal, and the coordinates X3 and X5respectively correspond to the positions of the start and end timepoints of actual X-ray exposure. In the embodiment, since times T1, T2,T4, and T6 are determined in advance, it is possible to calculate thevalues of X1, X2, X4, and X6 and set them in the apparatus in advance.Note however that it is possible to calculate the values of X1, X2, X4,and X6 for each measurement of an X-ray exposure delay in accordancewith the timing of switching of an X-ray exposure signal.

A vertical line 110 indicates a position in the flat panel detector 103at which pixel values are detected. The position of the vertical line110 in FIG. 2 is an example for facilitating the understanding of thedescription, and is not limited to the example.

(X-Ray Exposure in Moving Image Capturing Mode)

FIG. 3 is a timing chart showing an X-ray exposure signal, X-rayintensity, read start signal, charge read, and temporal transition ofread pixel value when X-ray exposure is performed once in the movingimage capturing mode. Times T1 to T6 correspond to the times when theX-coordinates X1 to X6 of the flat panel detector 103 shown in FIG. 2are read.

Referring to FIG. 3, the timing control unit 106 sets a read startsignal Hi at time T1, and sets an X-ray exposure signal Hi during chargereading. When the input read start signal is set Hi, the flat paneldetector 103 starts reading charges. When the input X-ray exposuresignal is set Hi, the X-ray generator 101 controls the X-ray tube 102 toemit X-rays. In addition, the timing control unit 106 sets the readstart signal Lo after the lapse of a predetermined time (between timesT1 and T2 in the case shown in FIG. 3), and sets the X-ray exposuresignal Lo during charge reading (time t4 in the case shown in FIG. 3).The flat panel detector 103 continues read operation until reading allthe rows without being influenced by the read start signal even if itgoes Lo. On the other hand, when the input X-ray exposure signal goesLo, the X-ray generator 101 stops X-ray exposure. In this manner, thetiming control unit 106 performs control to perform X-ray exposureduring charge reading. The period during which the timing control unit106 sets an X-ray exposure signal Hi varies depending on the X-rayimaging apparatus to be used. When using an X-ray imaging apparatusconfigured to perform moving image capturing at a frame rate of 30 FPS,this period is generally about several ms.

Referring to FIG. 3, like FIG. 7, “X-ray exposure signal” indicates asignal for performing X-ray exposure; “X-ray intensity”, the intensityof X-rays actually emitted; “read”, reading of charges from the flatpanel detector; and “pixel value”, pixel values on the vertical line 110of the flat panel detector in FIG. 2.

As described above, times T1 to T6 correspond to the times when theX-coordinates X1 to X6 of the flat panel detector 103 shown in FIG. 2are read. That is, time T1 is the time when charge reading starts, andis also the time when charge is read from the coordinate X1 of the flatpanel detector 103. Time T6 is the time when charge reading stops, andis also the time when charge is read from the coordinate X6 of the flatpanel detector 103. Time T2 is the time when the X-ray exposure signalis set Hi, and is also the time when charge is read from the coordinateX2 of the flat panel detector 103. When the X-ray exposure signal goesHi, the X-ray generator 101 emits X-rays from the X-ray tube 102. As hasbeen described, however, X-ray exposure actually starts from time T3 dueto an X-ray exposure start delay time. Time T3 is also the time whencharge is read from the coordinate X3 of the flat panel detector 103.Time T4 is the time when the X-ray exposure signal is set Lo, and isalso the time when charge is read from the coordinate X4 of the flatpanel detector 103. When the X-ray exposure signal goes Lo, the X-raygenerator 101 stops X-ray exposure from the X-ray tube 102. However, dueto an X-ray exposure stop delay time, X-ray exposure actually stops attime T5. Time T5 is the time when charge is read from the coordinate X5of the flat panel detector 103. Note, as described above, that thevalues of X1, X2, X4, and X6 are known in advance, and are set in theapparatus.

(Measurement of Delay Times Associated with X-Ray Exposure)

A procedure for measuring delay times associated with X-ray exposure bythe start delay measuring unit 108 and the stop delay measuring unit 109will be described next. The start delay measuring unit 108 and the stopdelay measuring unit 109 measure an X-ray exposure start delay time andan X-ray exposure stop delay time by analyzing an image obtained byX-ray exposure during charge reading.

More specifically, as shown in FIG. 3, the pixel value remains constantfrom time T1 when charge reading starts to time T3 when X-rays areactually emitted. After time T3, the pixel value increases until time T5when X-ray exposure actually stops. At this time, the pixel valuebecomes the value obtained by integrating X-ray intensities from time T3when X-rays are actually emitted to time T5 when charge reading starts.Subsequently, when X-ray exposure actually stops, the pixel valueremains constant until time T6 when pixel value reading ends.

As described above, charges are read from the flat panel detector 103row by row in a predetermined time (Tf in this embodiment). The value ofTf is known in advance, and is set in the apparatus. Letting T2 be thetime when an X-ray exposure signal is set Hi, and T4 be the time whenthe X-ray exposure signal is set Lo, the coordinates X2 and X4 read attimes T2 and T4 can be calculated byX2=X1+(T2−T1)/Tf  (1)X4=X1+(T4−T1)/Tf  (2)The start delay measuring unit 108 therefore calculates the X-rayexposure start delay time Ta according to the following equation usingthe coordinate X2 calculated by equation (1) and the coordinate X3corresponding to the time when the pixel value increases:Ta=(X3−X2)/Tf  (3)Likewise, the stop delay measuring unit 109 calculates the X-rayexposure stop delay time Tb according to the following equation usingthe coordinate X4 calculated by equation (2) and the coordinate X5 atwhich the pixel value becomes constant after an increase:Tb=(X5−X4)/Tf  (4)

As described above, this embodiment measures a delay associated withX-ray exposure by analyzing an image captured with uniform X-rays havingconstant intensity, in consideration of the fact that the charges storedin the detecting elements are read at a constant speed. That is, theembodiment measures delays based on the position of the boundary betweena region where there is a change in pixel value and a region where thereis no change in pixel value and the scanning speed associated withcharge reading. The embodiment, in particular, measures, as the abovedelay, the time required to scan, at the scanning speed, the distancebetween the boundary and a detecting element of the flat panel detectorwhich scans at the timing when the X-ray exposure signal (controlsignal) supplied from the control unit to the X-ray generator switches.According to the arrangement of the embodiment, therefore, it ispossible to measure delay times associated with X-ray exposure easily ata low cost without scanning any special device other than theconstituent elements of an existing X-ray imaging apparatus.

Although this embodiment has exemplified the case in which the followingtwo delay times are measured, one of the following may be a target to bespecified.

-   -   the delay from the instant an X-ray exposure signal switches        from a signal indicating the stop of outputting of X-rays to a        signal indicating outputting of X-rays to the instant the X-ray        generator actually starts outputting X-rays (X-ray exposure        start delay time); and    -   the delay from the instant an X-ray exposure signal switches        from a signal indicating outputting of X-rays to a signal        indicating the stop of outputting of X-rays to the instant the        X-ray generator actually stops outputting X-rays (X-ray exposure        stop delay time).

Referring to FIG. 3, a pixel value read for each row is the valueobtained by integrating X-ray intensities from time T3 when X-rays areactually emitted to the time when charges are read, and hence an X-rayintensity can be calculated by calculating a difference for each row.Displaying the calculated X-ray intensity, X-ray exposure signal, readsignal, pixel value, and the like on the display unit 104 allows tovisually check the timings of the signals as in a case in which they aremeasured by an oscilloscope.

The control unit 105 stores the measured X-ray exposure start delay timeTa and X-ray exposure stop delay time Tb in a storage device (notshown). It is possible to capture a good X-ray image by performingimaging by the technique disclosed in Japanese Patent Laid-Open No.62-276798 using these measured values at the time of general imagingoperation.

By using the above technique, the start delay measuring unit 108 canmeasure the X-ray exposure start delay time Ta, and the stop delaymeasuring unit 109 can measure the X-ray exposure stop delay time Tb.

This embodiment has exemplified the arrangement in which the delay timemeasuring unit 107 includes both the start delay measuring unit 108 andthe stop delay measuring unit 109. The embodiment may be configured toinclude one of the start delay measuring unit 108 and the stop delaymeasuring unit 109.

This embodiment has exemplified the case in which the read sequence ofthe detecting elements coincides with the downward direction of the flatpanel detector 103. However, the present invention is not limited tothis. For example, it is possible to read charges from both sides of theflat panel detector 103, that is, downward and upward, or to readcharges in a horizontal direction. In addition, the embodiment hasexemplified the arrangement for driving the flat panel detector.However, the present invention is not limited to this. For example, itis possible to use a MIS photodiode.

In this embodiment, the pixel values shown in FIG. 3 are described asthose on the vertical line 110 shown in FIG. 2. However, the presentinvention is not limited to this. For example, it is possible to use anaverage pixel value obtained by averaging pixel values for each row.Furthermore, the embodiment has exemplified the case in which thecontrol unit 105 stores the measured X-ray exposure start delay time Taand X-ray exposure stop delay time Tb. However, the present invention isnot limited to this. For example, the X-ray generator 101 or the flatpanel detector 103 may store them.

In addition, according to the above description, the timing control unit106 sets an X-ray exposure signal Hi for about several ms. However, thepresent invention is not limited to this. For example, the timingcontrol unit 106 may set the X-ray exposure signal Hi for the time inputby an input device (not shown). Furthermore, the embodiment hasexemplified the case in which the control unit 105 incorporates thetiming control unit 106. However, the present invention is not limitedto this. For example, the X-ray generator 101 or the flat panel detector103 may incorporate the timing control unit 106.

The second embodiment of the present invention will be described next.An X-ray imaging apparatus according to this embodiment has anarrangement similar to that of the X-ray imaging apparatus of the firstembodiment, in which a timing control unit 106 performs control to emitX-rays during charge reading as in the first embodiment. This embodimentdiffers from the first embodiment in that it emits X-rays a plurality ofnumber of times, and performs a read for offset correction, a readduring X-ray exposure, and a read for resetting detecting elements.

FIG. 4 is a timing chart showing an X-ray exposure signal, X-rayintensity, read start signal, read, and read pixel value in the movingimage capturing mode. Referring to FIG. 4, reference symbol RD denotes aread without X-ray exposure; RX, a read with X-ray exposure; and RR, aread for resetting charges stored in detecting elements. Referencesymbols RD1, RX1, and RR1 each denote the first read; and RD2 and RX2each, the second read. A pixel value In (n is a natural number equal toor more than 1) of the nth offset-corrected image is obtained byequation (5). Note that RXn and RDn each indicate a pixel value read bythe nth read.In=RXn−RDn(n=1, 2, . . . )  (5)

A delay time measuring unit 107 including a start delay measuring unit108 and a stop delay measuring unit 109 measures an X-ray exposure startdelay time and an X-ray exposure stop delay time by analyzing theoffset-corrected image In. A measurement procedure is the same as thatin the first embodiment, and hence a description of it will be omitted.

In addition, the delay time measuring unit 107 performs X-ray exposure aplurality of number of times to measure an X-ray exposure start delaytime and an X-ray exposure stop delay time a plurality of number oftimes. Of the X-ray exposure start delay times and X-ray exposure stopdelay times measured a plurality of number of times, the maximummeasured values are set as an X-ray exposure start delay time and anX-ray exposure stop delay time.

In addition, times T41 to T43 each are the time when charge is read froma coordinate X4 in FIG. 2, time T41 is the time during the read RX1,time T42 is the time during the read RD1, and time T43 is the timeduring the read RD2. The detecting element (the detecting element at thecoordinate X4) from which charge is read at time T41 is reset after thecharge is read. However, since X-ray exposure continues after time T41,the detecting element at the coordinate X4 detects X-rays again andstores charge. For this reason, when charge is read from the detectingelement at the coordinate X4 at time T42, the pixel value obtained isthe one obtained by X-ray exposure. When charge is read from thedetecting element, the charge stored in the detecting element is resetthereafter. When, therefore, charge is read from the detecting elementat the coordinate X4 at time T43, the obtained pixel value is the oneobtained without X-ray exposure. For this reason, the image obtained bythe read RD2 can be used as an offset-corrected image.

When performing offset correction a plurality of number of times in thismanner, it is necessary to perform the read RD without X-ray exposure,the read RX with X-ray exposure, and the read RR for resetting thecharge stored in the detector.

As described above, this embodiment measures delays associated withX-ray exposure operation by analyzing the difference image based on animage generated when X-rays are output and an image captured when noX-rays are output. Since the embodiment performs delay measurement byoffset correction in this manner, it is possible to reduce measurementerrors.

This embodiment performs offset correction by using equation (5).However, the present invention is not limited to this. For example, theread RD1 without X-ray exposure may be performed only once, and thesecond or subsequent reads RD2 without X-ray exposure may be omitted. Inthis case, the offset-corrected image In is calculated byIn=RXn−RD1(n=1, 2, . . . )  (6)

This embodiment has exemplified the maximum values of measured values asan X-ray exposure start delay time and an X-ray exposure stop delaytime. However, the present invention is not limited to this. Forexample, it is possible to use values obtained by adding times input viaan input device (not shown) to the maximum X-ray exposure start delaytime and X-ray exposure stop delay time.

The third embodiment of the present invention will be described next. AnX-ray imaging apparatus according to this embodiment has an arrangementsimilar to that of the X-ray imaging apparatus according to the firstembodiment, in which a timing control unit 106 performs control to emitX-rays during charge reading as in the first embodiment. This embodimentwill exemplify a case in which an X-ray exposure time is long, and hencethe time interval from the start of one X-ray exposure to the end of theX-ray exposure does not fall within a moving image read period.

FIG. 5 is a timing chart showing an X-ray exposure signal, X-rayintensity, moving image read, still image read, and read pixel value inthe moving image capturing mode. An illustration of “read start signal”will be omitted because it has already been described.

Referring to FIG. 5, time T3 is the time when X-ray exposure actuallystarts, and time T5 is the time when X-ray exposure actually stops. Inthe case shown in FIG. 5, the X-ray exposure start delay time and theX-ray exposure stop delay time are long, and hence times T3 and T5 donot fall within a moving image read period. In this case, a still imageread with a long read period is performed to measure an X-ray exposurestart delay time and an X-ray exposure stop delay time. Referring toFIG. 5, since a still image read period is long, times T3 and T5 fallwithin a still image read period. It is therefore possible to measure anX-ray exposure start delay time and an X-ray exposure stop delay time byusing the above technique. Note, however, that since a still image readis performed in a time Tr different from a moving image read time Tf foreach row, it is necessary to perform calculation according to equations(1) to (4) upon replacing Tf with Tr.

As described above, according to this embodiment, in the arrangementconfigured to read charges from the detecting elements at predeterminedtime intervals in a predetermined read period, when the time intervalfrom the start of one X-ray exposure to the end of the X-ray exposuredoes not fall within a read period, the read period is prolonged (in thestill image read period). This embodiment can therefore measureoperation delays associated with X-ray exposure regardless of theenvironment in which the X-ray imaging apparatus performs moving imagecapturing.

This embodiment has exemplified a read in a long read time as a stillimage read. However, the present invention is not limited to this. Sincea charge read time needs to be long, the time during which charges areread from one row of the flat panel detector may be set to a time otherthan the time Tr.

In addition, this embodiment has exemplified the case in which a movingimage read is performed first, and a still image read is then performed.However, the present invention is not limited to this. For example, itis possible to perform a still image read in a long read time first.

The fourth embodiment of the present invention will be described next.An X-ray imaging apparatus according to this embodiment has anarrangement similar to that of the X-ray imaging apparatus according tothe first embodiment, in which a timing control unit 106 performscontrol to emit X-rays during charge reading as in the first embodiment.The embodiment will exemplify a case in which an X-ray exposure time islong, and the time interval from the start of one X-ray exposure to theend of the X-ray exposure extends across a plurality of moving imageread periods.

FIG. 6 is a timing chart showing an X-ray exposure signal, X-rayintensity, moving image read, and read pixel value at the time of movingimage capturing. An illustration of “read start signal” will be omittedbecause it has already been described. An illustration of “read” withoutX-ray exposure for offset correction will be omitted because it hasalready been described in the second embodiment.

Referring to FIG. 6, reference symbol RX denotes a read with X-rayexposure; and RR, a read for resetting the charges stored in thedetector. Reference symbols RX1 and RR1 each denote the first read; andRX2 and RR2 each, the second read. An illustration of “read RD” shown inFIG. 4 will be omitted because it has already been described. Time T11is the time when the first read RX1 starts; time T12, the time when thefirst read RR1 starts; T13, the time when the second read RX2 starts;time T2, the time when an X-ray exposure signal is set Hi; time T3, thetime when X-ray exposure actually starts; time T4, the time when theX-ray exposure signal is set Lo; and time T5, the time when X-rayexposure actually stops.

Referring to FIG. 6, since the X-ray exposure start delay time and theX-ray exposure stop delay time are long, times T3 and T5 do not fallwithin one moving image read period. In this case, a moving image readis performed a plurality of number of times to measure X-ray exposurestart delay times and X-ray exposure stop delay times. In addition,times T3 and T5 respectively fall within the first and second reads RX1and RX2 upon adjustment of time T2 when the X-ray exposure signal is setHi, time T11 when the first read RX1 starts, and time T4 when the X-rayexposure signal is set Lo.

A start delay measuring unit 108 calculates an X-ray exposure startdelay time Ta according to equation (7) using a coordinate X3 at whichthe pixel value increases:Ta=T11−T2+(X3−X1)*Tf  (7)

Likewise, the stop delay measuring unit 109 calculates an X-ray exposurestop delay time Tb according to equation (8) using a coordinate X5 atwhich the pixel value becomes constant after a coordinate X4 calculatedby equation (2) described in the first embodiment is calculated, and thepixel value increases:Tb=T13−T12−(X4−X1)*Tf+(X5−X1)*Tf  (8)

As described above, this embodiment has exemplified the case in whichthe timing at which the X-ray exposure signal switches and the timing atwhich a detecting element existing at the boundary associated with thepresence/absence of variation in pixel value in the captured image isoperated for scanning respectively exist in different read periods. Insuch a case, as well as in the first embodiment, it is possible tomeasure operation delays associated with X-ray exposure.

Each arrangement described above can be applied to X-ray imagingapparatuses, more specifically X-ray imaging apparatuses as medicalX-ray imaging apparatuses and industrial nondestructive examinationapparatuses. As described above, the arrangement of each embodiment isconfigured to measure an X-ray exposure start delay time and an X-rayexposure stop delay time by performing X-ray exposure during a read andanalyzing an X-ray image captured during X-ray exposure. This makes itpossible to easily measure at least one of an X-ray exposure start delaytime and an X-ray exposure stop delay time without increasing the cost.

The present invention can provide a technique of easily measuring delaytimes occurring at the start and stop of X-ray exposure at a low costwithout providing any special arrangement.

Other Embodiments

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (for example, computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2010-172728, filed on Jul. 30, 2010, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An X-ray imaging apparatus which obtains an X-ray image, the apparatus comprising: an imaging unit including a plurality of detecting elements adapted to convert X-rays generated by an X-ray generating apparatus which outputs or stops X-rays in accordance with an operation instruction into an image signal; and an obtaining unit adapted to obtain an operation start timing of the X-ray generating apparatus based on an image signal output from said imaging unit and obtain a difference between a timing of an operation instruction to the X-ray generating apparatus and the operation start timing.
 2. The apparatus according to claim 1, further comprising a generating unit adapted to generate an image by sequentially operating the plurality of detecting elements for scanning at a constant speed and converting an image signal read from each detecting element into a pixel value, wherein said obtaining unit obtains, as the difference, a delay time of operation associated with outputting of X-rays from the X-ray generating apparatus relative to a timing of inputting of an operation signal to the X-ray generating apparatus by analyzing the generated image.
 3. The apparatus according to claim 2, wherein said obtaining unit obtains the delay time based on a position of a boundary between a region where there is a change in pixel value in the image and a region where there is no change in pixel value and a speed of the scanning.
 4. The apparatus according to claim 3, wherein said obtaining unit obtains, as the delay time, a time required to scan, at the speed of the scanning, a distance between the boundary and the detecting element operated to scan at a timing of inputting of the operation signal.
 5. The apparatus according to claim 1, wherein said obtaining unit obtains a difference between the instant the operation instruction is issued and the instant the X-ray imaging apparatus actually starts outputting X-rays and a difference between the instant the operation instruction is issued and the instant the X-ray imaging apparatus actually stops outputting X-rays.
 6. The apparatus according to claim 2, wherein said obtaining unit obtains the delay time by analyzing a difference image based on an image generated by said generating unit and an image captured when the X-rays are not output.
 7. The apparatus according to claim 1, further comprising a reading unit adapted to sequentially operate the plurality of detecting elements for scanning at a constant speed and read image signals from the respective detecting elements, wherein said reading unit reads the image signal at predetermined time intervals in a predetermined read period, and reads the image signal upon prolonging the read period when a time interval from a start of one X-ray exposure to an end of the X-ray exposure does not fall within the read period.
 8. The apparatus according to claim 1, further comprising a reading unit adapted to sequentially operate the plurality of detecting elements for scanning at a constant speed and read image signals from the respective detecting elements, wherein said reading unit reads the charge at predetermined time intervals in a predetermined read period, and reads the charge upon prolonging the read period when a time interval from a start of one X-ray exposure to an end of the X-ray exposure does not fall within the read period.
 9. The apparatus according to claim 1, further comprising a control unit adapted to control a storage start timing of said imaging unit based on the obtained difference.
 10. A method of measuring a delay time of X-ray exposure by an X-ray imaging apparatus which obtains an X-ray image, the method comprising the steps of: causing an X-ray generating apparatus to change an X-ray output state in accordance with an operation signal; sequentially reading image signals from an imaging unit adapted to convert the X-rays into image signals; and measuring a delay of operation of the generating apparatus relative to the operation signal by analyzing the image signal. 